Unique key constraints in Azure Cosmos DB.; 3 minutes to read +6; In this article. Unique keys add a layer of data integrity to an Azure Cosmos container. You create a unique key policy when you create an Azure Cosmos container. With unique keys, you make sure that one or more values within a logical partition is unique. UUID, Universal Unique Identifier, is a python library which helps in generating random objects of 128 bits as ids. It provides the uniqueness as it generates ids on the basis of time, Computer hardware (MAC etc.). Advantages of UUID: Can be used as general utility to generate unique random. Instead you can use a single key and, as value, a list of elements that had that key. So you can follow these steps: See if the current element's (of your initial set) key is in the final dict. If it is, go to step 3. Update dict with key. Append the new value to the dictkey list. While there are many sophisticated ways to generate these, in many cases it is perfectly adequate to use sufficiently long and random sequences of characters. The problem is that if you are doing this in Python, there is more than one way to generate random strings, and it isn't always clear which way is the best and most secure.
SQL UNIQUE Constraint
The UNIQUE constraint ensures that all values in a column are different.
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Both the UNIQUE and PRIMARY KEY constraints provide a guarantee for uniqueness for a column or set of columns.
A PRIMARY KEY constraint automatically has a UNIQUE constraint.
However, you can have many UNIQUE constraints per table, but only one PRIMARY KEY constraint per table.
SQL UNIQUE Constraint on CREATE TABLE
The following SQL creates a UNIQUE constraint on the 'ID' column when the 'Persons' table is created:
SQL Server / Oracle / MS Access:
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CREATE TABLE Persons (
ID int NOT NULL UNIQUE, LastName varchar(255) NOT NULL, FirstName varchar(255), Age int );
MySQL:
CREATE TABLE Persons (
ID int NOT NULL, LastName varchar(255) NOT NULL, FirstName varchar(255), Age int, UNIQUE (ID) );
To name a UNIQUE constraint, and to define a UNIQUE constraint on multiple columns, use the following SQL syntax:
MySQL / SQL Server / Oracle / MS Access:
CREATE TABLE Persons (
ID int NOT NULL, LastName varchar(255) NOT NULL, FirstName varchar(255), Age int, CONSTRAINT UC_Person UNIQUE (ID,LastName) ); SQL UNIQUE Constraint on ALTER TABLE
To create a UNIQUE constraint on the 'ID' column when the table is already created, use the following SQL:
MySQL / SQL Server / Oracle / MS Access:
To name a UNIQUE constraint, and to define a UNIQUE constraint on multiple columns, use the following SQL syntax:
MySQL / SQL Server / Oracle / MS Access:
ALTER TABLE Persons
ADD CONSTRAINT UC_Person UNIQUE (ID,LastName); DROP a UNIQUE Constraint
To drop a UNIQUE constraint, use the following SQL:
MySQL:
SQL Server / Oracle / MS Access:
All DDL (Data Definition Language) statements are executed using a handle structure known as a cursor. The following examples show how to create the tables of the Employee Sample Database. You need them for the other examples.
In a MySQL server, tables are very long-lived objects, and are often accessed by multiple applications written in different languages. You might typically work with tables that are already set up, rather than creating them within your own application. Avoid setting up and dropping tables over and over again, as that is an expensive operation. The exception is temporary tables, which can be created and dropped quickly within an application.
The preceding code shows how we are storing the
CREATE statements in a Python dictionary called TABLES . We also define the database in a global variable called DB_NAME , which enables you to easily use a different schema.
A single MySQL server can manage multiple databases. Typically, you specify the database to switch to when connecting to the MySQL server. This example does not connect to the database upon connection, so that it can make sure the database exists, and create it if not:
We first try to change to a particular database using the
database property of the connection object cnx . If there is an error, we examine the error number to check if the database does not exist. If so, we call the create_database function to create it for us.
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On any other error, the application exits and displays the error message.
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After we successfully create or change to the target database, we create the tables by iterating over the items of the
TABLES dictionary:
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To handle the error when the table already exists, we notify the user that it was already there. Other errors are printed, but we continue creating tables. (The example shows how to handle the “table already exists” condition for illustration purposes. In a real application, we would typically avoid the error condition entirely by using the
IF NOT EXISTS clause of the CREATE TABLE statement.)
The output would be something like this:
To populate the employees tables, use the dump files of the Employee Sample Database. Note that you only need the data dump files that you will find in an archive named like
employees_db-dump-files-1.0.5.tar.bz2 . After downloading the dump files, execute the following commands, adding connection options to the mysql commands if necessary:
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